The invention relates to a method for reading a set of tags, each with an ID code and capable of randomly going into or out of an electromagnetic field transmitted by a polling device. It also relates to a system implementing this method.
The invention is applied in the field of recognizing persons bearing badges, or in the fields of counting and controlling objects bearing tags, such as luggage at an airport, goods in a store, or else products in a production line.
Professionals presently know numerous systems and methods for identifying tagged objects. Most of them apply to reading multiple tags.
Some of these methods provide retransmission of the tag code, after a random delay peculiar to each tag, when a collision of messages being transmitted simultaneously by the tags is detected.
There are also methods consisting in leaving a particular time slice for a tag""s response. Each time slice is determined uniquely by the ID code of each tag. However, these methods do not optimize the transaction time between the polling system and the set of tags. Moreover, the time required by the method for reading all the tags can be based on the drawing of random numbers in addition to the hazard of the number of tags present which implies that it is not deterministic.
Another method, only adapted for a static set of tags to be read once, is described in the French patent FR-A-2 677 135. This patent application explains how the polling system makes the tags present in the polling field of said system successively supply each bit of their ID code until it is entirely identified. For this purpose, the tags respond to a control signal from the polling device; when a tag detects that the code currently being identified is different from its own, it temporarily locks up (i.e. it goes xe2x80x9csilentxe2x80x9d) so that the identification cycle continues with the other tags until there is only one unlocked tag left; the code of this tag is then identified. At the end of the identification cycle, upon a single command from the polling device, the identified tag locks up permanently and the other tags end their temporary locking. The identification method is then reinitialized for identifying another tag. These operations are repeated as many times as required for identifying all the tags individually.
However, this technique can only be applied to a static set of tags that can only be read once. Therefore, it cannot be applied to a dynamic set of tags, i.e. to tags that can randomly go into and out of the electromagnetic field of the polling system.
Furthermore, the polling time of the tags according to this method is all the longer as the number of tags is high; consequently, this method is rather lengthy, once the number of tags is high and it is desired to restart a new read cycle by temporarily removing the electromagnetic field transmitted by the polling device to cancel the permanent locking of the tags already identified.
It is precisely the object of the invention to remedy the disadvantages of the previously mentioned techniques. For this purpose, it provides a method and system for reading a dynamic set of tags present in the electromagnetic field of a polling device, where the time for checking the presence of the tags already identified is minimized by assigning to each of these tags a sequence number whose binary size is significantly smaller than that of the ID code.
More precisely, the invention relates to a method for reading a dynamic set of tags, each with a distinct ID code and located in the electromagnetic field of a polling device, consisting in identifying the tags by successively reading their codes by means of transmitting and receiving signals between the polling device and the tags. This method is characterized by consisting in carrying out a first read cycle during which each identified tag is locked and during which a sequence number is assigned to each of the identified tags so as to enable the creation of a list of all the tags available in the field of the polling device, then during subsequent read cycles, carrying out an update of this list by reactivating the already listed tags, by checking from the sequence number of these tags those that are still present in the field and removing those that have left it and adding the tags that have just entered it.
Advantageously, when updating the list of tags, the sequence numbers of the tags are modified depending on when the tags go out of and into the field, so that the sequence numbers of the list are consecutive.
According to the invention, the sequence number of each tag is determined, in parallel, by the tag itself and by the polling device, from the information exchanged by the tags and said polling device when identifying the tags.
In this case, the sequence number can be initialized automatically at the polling device and at each tag.
The invention also relates to a system for reading a dynamic set of tags via a polling device, implementing the above method and wherein the tags and the polling device each comprise signal transmitting/receiving means and an ID code storage memory. This reading system is characterized by the polling device comprising:
a pointer for pointing to its storage memory;
a counter for counting the number of tags present; and
a register for storing the number of current tags;
it is also characterized by each tag comprising:
a first ID code read status latch;
a temporary lock latch and a cyclic lock latch;
a storage register for its sequence number;
a sequence number counter; and
a sequence number decounter.